A plunger tip for use in aluminum die casting is a closed end metal cylinder having an axial cavity therein and a shank of smaller diameter at the other end. The shank is connectable to a control rod or the like, as by having a threaded bore. Typically, the plunger tip is made of cast beryllium-copper alloy.
Purchasers of the plunger tips frequently machine the cylinders to a slightly smaller outside diameter. On occasion the machining operation may reduce the wall thickness of the cylinder to such a degree that the wall will crack or break during use, thus resulting in failure of the plunger tip.
According to the present invention the plunger tip is provided with an indicator surface which indicates the extent to which the cylinder can be reduced in diameter without reducing the wall thickness to a dimension which might cause failure of the plunger tip during use. The indicator surface is an external outwardly facing surface on the plunger tip located intermediate the inside and outside diameters of the cylinder. In other words the outwardly facing indicator surface lies at a distance from the axis of the plunger tip which is equal to one-half the maximum safe outside diameter of the cylinder after machining. When the cylinder of plunger tip is machined (by mounting the tip in a lathe, rotating it about its axis and cutting away metal from the cylinder), the outside diameter is reduced to not less than the outside diameter of the rotational path of the indicator surface. The relationship between this path and the outside diameter of the cylinder is easily determined by the machinist by observation during the machining operation.
When the junction between the end of the cylinder and the shank is located at a shoulder, as is typically the case, the safety indicator surface may be conveniently the outer surface of a projection located on the shank immediately adjacent the shoulder because this allows the machinist to easily compare the outside diameter of the cylinder with the rotational path of the indicator projection. The indicator projection is preferably cast integrally with the shank of the plunger tip. It can, however, be secured to the shank subsequent to casting. The shape and number of projections is not critical. When the shank is cylindrical the projection may be a circumferential bead. If the shank is hexagonal in cross-section or otherwise has flat surfaces, one or more of the flat surfaces may be provided with a small boss.
The indicator surface may also be all or part of the external surface of the shank, without there being provided a special projection. For example, if the shank is hexagonal or square or otherwise has flat surfaces which intersect each other at lines located at the maximum diameter of the shank, any one of the lines of intersection can be the indicator surface provided that the cross-sectional size and shape of the shank is such that the line occurs at the desired distance from the axis of the tip.
The indicator surface may also be a surface on the cylinder itself. For example, the surface can be formed by an annular shoulder on the cylinder at one end thereof, such as the end adjacent the shank. The surface can also be formed by the bottom of an annular groove in the cylinder or the bottom of one or more recesses in the cylinder.